1. Field of the Invention
This invention relates to U.V.stabilized polyvinyl chloride articles, and, more particularly, to polyvinyl chloride articles having a surface modified effectively with a U.V. stabilizer.
2. Description of the Prior Art
Titanium dioxide, which is a white pigment, has a dual function in polyvinyl chloride articles. The first function of titanium dioxide is to stabilize polyvinyl chloride against the adverse effects of U.V. radiation by absorbing, reflecting, dispersing or scattering the incident U.V. energy. The second function of titanium dioxide is to render the article opaque so that colored articles may be produced, if desired, by including colorants therein in an amount sufficient to overcome the white titanium dioxide present.
Unfortunately, the amount of titanium dioxide necessary for effective U.V. stabilization, about 12 phr, precludes deep coloration of such articles. In fact, only colored articles of pastel shades can be obtained; deep shades of brown, blue and maroon, for example, are not readily attainable. Accordingly, it would be advantageous to provide U.V. stabilized polyvinyl chloride articles containing titanium dioxide in amounts only necessary to render the article opaque.
Of course, the use of low concentrations of titanium dioxide is accompanied by the loss of its U.V. stabilization function. Therefore, for low titanium dioxide articles, it is necessary to provide an alternative means of U.V. stabilization for such articles. Since titanium dioxide is such a cheap raw material, it is uneconomical to substitute an effective, but high priced organic U.V. stabilizer, such as a benzophenone or benzotriazole, in place of titanium dioxide. Furthermore, since photodegradation is known to be a surface phenonenon, bulk incorporation of such organic U.V. stabilizers is inefficient because the stabilizer is present substantially in the interior of the article, where it does not act to prevent surface photodegradation.
For these reasons, surface treatments of polyvinyl chloride articles containing titanium dioxide have been proposed to provide effective and economical U.V. stabilization for polyvinyl chloride resins. These prior art surface treatments are based on the ability of various organic solvents to swell the resin so that a solution of the U.V. stabilizer can be directed into the surface regions of the article, where protection is desired. However, none of these surface treatments have been very successful commercially.
For example, Jochanan, in Israeli Patent No. 39037, published Nov. 10, 1975, entitled "U.V. - and Oxidation-Proof Products from Organic Plastics and their Manufacture", describes various methods of applying a solution or suspension of a U.V. absorber onto the surface of a plastic sheet containing titanium dioxide. The methods disclosed by Jochanan include dipping the resin sheet into a solution of the stabilizer, or spraying, brushing, roller-printing or curtain-coating the solution onto the sheet. With such methods, however, a large amount of residual stabilizer and solvent is left on the surface of the article, and the modified surface is observed to be streaky, hazy, and has runs or pockmarks thereon.
Other attempts at effective surface modification of polyvinyl chloride are described by Katz, et al, in the "Proceedings of the A.C.S. Div. of Org. Coatings and Plastics" 36, (1) p. 202-205(1976); Katz, et al., Soc. Plast. Eng., Tech. Papers (1976).sup.22, 511-512; U.S. Pat. Nos. 3,043,709; 3,297,462; 3,519,462, 3,783,011; 3,892,889; 4,146,658; 4,323,597; 4,349,067; and Belgium Patent No. 612,206. However, none of these disclosed processes have provided useful surface modified, U.V. stabilized, opaque polyvinyl chloride resin articles, for use, for example, in residential siding, which application requires particularly effective protection from the weathering effects of sunlight, and an appearance and degree of surface perfection which is acceptable to the public.
Accordingly, an object of the present invention is to provide a surface modified, U.V. stabilized polyvinyl chloride resin article containing titanium dioxide in an amount sufficient to render it opaque.
Another object of the invention is to provide a resin sheet article wherein the modified surface is substantially free of residual stabilizer and solvent materials, and the appearance and degree of surface perfection of the modified side is substantially the same as that of the unmodified side of the sheet.
Another object herein is to provide such resin articles wherein the amount of stabilizer within the modified surface region of the sheet is at least 0.3 g/m.sup.2 of the surface area of the sheet, and, most preferably, 0.5 to 10.7 g/m.sup.2, where the surface region is defined as a depth extending up to about 200 microns from the surface, and at least 70% of this stabilizing amount is present within the first 100 microns of the surface region.
Still another object of the invention is to provide the desired U.V. stabilization within the surface region with polyvinyl chloride resin articles containing titanium dioxide in an amount of at least 0.5 phr, preferably less than about 8 phr, and, most preferably, about 1 to 6 phr.
Yet another object herein is to provide a method of making such PVC articles in which the treated PVC sheet is protected from the atmosphere during formation of the surface modified article.
These and other objects, features and advantages of the present invention will be readily apparent from the following detailed description thereof, which is to read in connection with the accompanying drawings, in which: